FACTORS INFLUENCING WATER QUALITY PARAMETERS IN A LINEAR CLUSTER OF THREE PUBLIC WATER WELLS WITHIN A SMALL AQUIFER, EASTERN MASSACHUSETTS

A linear array of three water supply wells in southeastern Massachusetts, 0.20 and 0.38 mi apart respectively, yields a wide range of quality of raw water. Iron levels at one end of the array are near 0.05 ppm increasing to 1 to 2 ppm in the middle and continuing to 10 to 15 ppm at the opposite end, approximately an increase by factor 300 with similar, although less dramatic, trends noted for other components as well. The well array transects a small, 1.0 by 0.5 mi, aquifer formed by a glacial pitted outwash plain filling a buried narrow valley with its bottom elevation below sea level. Glacial deposits filled only about 75% of the volume with the rest filled by lacustrine peat deposits. Monitoring wells on both sides of the transition zone yield data that show a presence of two contrasting hydrochemical domains controlled by the respective equilibria within their environments. The outwash plain is an elevated small plateau (upland) approximately 10 to 15 feet above the peat surface (wetlands). Ground water in the upland part of the aquifer has low dissolved solids, high oxidation potential and pH values ranging between 6.0 and 6.5 whereas ground water within the wetlands has higher load of dissolved solids, strongly reducing potential, and near neutral or slightly above 7 pH. Hydrochemistry of ground water within the uplands is influenced largely by chemical weathering of biotite and feldspar grains (derived from local granites during the glacial period) in contrast to ground water in the peat section where decay of organic material and release of organically bound components are the dominating factors. The array of wells runs toward the boundary separating the two hydrochemical domains. The determining factor for the wide range of water quality parameters during pumping is the mixing ratio of the fluxes of waters drawn from the respective reservoirs. Our data also indicate that ground water mixing along the mixing front may lead to a formation of suspended iron hydroxide solids.